미국 지리학 조사 협회(USGS)가 새로 발간한 보고서는 자국 내 하천들의 수질이 물의 흐름을 바꿔놓고 유기물과 농약의 수치를 상승시킨 외부 환경적 요인들로 인해 급격히 저하되었다고 설명하고 있다.

위 보고서는 다양한 생태 환경에 대한 평가, 유역의 흐름 변화, 100개가 넘는 물의 함유 성분, 하천 퇴적물의 측정 등을 포함하고 있다. 더 나아가, USGS의 과학자들은 세 개의 생물의 식생(algae, macroinvertebrates and fish)과 인간이 자초한 인공의 환경적 변화들 간의 관계를 설명한다. 이와 같이 하천이 생태 환경을 뒷받침할 수 있는 능력이 된다는 것은 하천 자체의 건강을 대신 보여주는 것과 같다고 할 수 있다.

July 11, 2013 -- A new report from the U.S. Geological Survey (USGS) explains how the health of U.S. streams is being degraded by streamflow modifications and elevated levels of nutrients and pesticides.

The report included assessments of multiple biological communities as well as streamflow modifications and measurements of over 100 chemical constituents in water and streambed sediments. Further, USGS scientists examined the relationship of the condition of three biological communities (algae, macroinvertebrates and fish) to man-made changes in streamflow characteristics and water quality. The ability of a stream to support these biological communities is a direct measure of stream health.

"Healthy streams are an essential part of our natural heritage. They are important to everyone -- not only for recreation and for public water supply and public health, but also for economic growth," said USGS acting Director Suzette Kimball. "A broad understanding of the complex factors that affect stream health across the nation will aid us in making efficient, long term decisions that support healthy streams."

Stream health was reduced at the vast majority of streams assessed in agricultural and urban areas. In these areas, at least one of the three aquatic communities was altered at 83 percent of the streams assessed. In contrast, nearly one in five streams in agricultural and urban areas was in relatively good health, signaling that it is possible to maintain stream health in watersheds with substantial land and water-use development.

Annual low and high streamflows were modified in 86 percent of the streams assessed. Over 70,000 dams and diversions contribute to modified streamflows across the Nation. Flood control structures in the East and groundwater withdrawals for irrigation and drinking water in the arid West also contribute to streamflow modification.

Biological alteration associated with elevated nutrient concentrations was most pronounced for algal communities. The occurrence of altered algal communities increased by as much as 40 percent above baseline in streams with elevated nitrogen and phosphorus concentrations.

Macroinvertebrate communities were altered by as much as 40 percent above baseline conditions in streams with elevated pesticide toxicity. Although concentrations of insecticide mixtures, such as chlorpyrifos, carbaryl, and diazinon, in streams are highly variable seasonally and from year to year, they can reach levels that are harmful to aquatic life, particularly in agricultural and urban streams.

This study was done by the USGS National Water-Quality Assessment Program, which conducts regional and national assessments of the nation’s water quality to provide an understanding of water-quality conditions, whether conditions are getting better or worse over time, and how natural processes and human activities affect those conditions.

[출처: Waterworld / 2013년 7월 11일]